1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to speed pattern generators for an elevator car.
2. Description of the Prior Art
The speed of an elevator car is normally controlled by a speed pattern in a traction elevator system. The speed pattern includes four major portions: acceleration, full speed, deceleration, and landing. The landing speed pattern provides the transition between full or maximum deceleration to zero deceleration. The landing speed pattern can be generated digitally from the distance-to-go (DTG) or by analog devices such as hatch inductors or transducers.
A linear landing pattern would provide the fastest landing. However, it would land the car at full deceleration and zero velocity, and the rapid change from full deceleration to zero deceleration would provide a large jerk which would greatly exceed the maximum passenger comfort level. Thus, the landing speed pattern which is usually used in exponential in nature, with the DTG being an exponential function of time, starting from the point at which the landing is initiated, as expressed by the following relationship: EQU DTG=Ke.sup.-t /.tau. (1)
where:
t=time PA1 .tau.=a time constant related to the selected landing distance, and the selected velocity at that distance. PA1 P=landing pattern PA1 v=car velocity PA1 a=deceleration rate PA1 T=system lag to a ramp input
The car velocity, acceleration and jerk during landing can be derived from equation (1) by taking successive derivatives thereof, and they are also exponential in nature.
During acceleration the actual speed of the elevator car lags the speed pattern, and during deceleration and landing the actual speed is greater than the speed pattern, because of the system time delay. The closed loop speed transfer function of an elevator motor controller can be approximated by a slightly underdamped second order system which will follow a time ramp with a constant delay of T, which is normally about 0.25 sec.
Since the landing pattern is lagging the car landing velocity, the landing pattern can be expressed as: EQU P(t)=v(t)-a(t)T (2)
where:
A landing speed pattern derived from equation (2) provides a reasonably good landing and a comfortable ride for the passengers. However, it does have some disadvantages. Being exponential in nature, the landing speed pattern produces a non-zero final car speed, and it requires in excess of two seconds to land the car.